Hi,

Following various softwares are used for combination index calculation. (COMPUSYN, CalcuSyn, synergy - Biostatistics) try to download and use.

or

for manual calculations read this article:

Liang Zhao, Jessie L.-S. Au, and M. Guillaume Wientjes, Comparison of methods for evaluating drug-drug interaction, Front Biosci (Elite Ed). ; 2: 241–249), 2010.

The combination index can be calculated easily by the fractional product method of Webb: i.e.first, calculate the so called "predicted value" (c) by using the equation

c = a x b/100 where "a" and "b" indicate cell survival values with single agents. Indeed, the "c" value represent the effect that you would expect if the compounds did not interact at all i. e. in case of an additivive effect. Hence, Combination Index C.I. can be obtained as the ratio between survival observed with the combination in your experimental sample and the predicted value. Thus, a C.I. equal to 1, less than 1 or more than 1 will indicate additive effects, synergism and antagonism respectively.

Use Compu Syn software to calculate combination index for drugs. This software is very easy to use and tells u about synergism, antagonism and additive effects of drug combination

Sandeep, Is there  a compusyn user guide - available for download?